1. Field of the Invention
The present invention relates to horizontal scanning circuits of electron tube displays. The present invention more specifically relates to compensation circuits of electron beam deflection.
2. Discussion of the Related Art
A well known issue of electron beam deflection is the “breathing” phenomenon. Bright images are wider than dark ones. In fact, bright images require a higher electron beam current than dark images. When the electron beam current increases, the high voltage (HV) of the electron tube display accelerating the electron beam decreases due to the power supply internal resistance. As the acceleration of the electrons decreases, their deflection increases. Bright video displays are then wider (more deviated to the right and to the left) than dark video displays.
Known breathing compensation circuits include HV measuring circuits. Scanning circuits are adapted to change the deflection coil current according to the measured HV variation so as to compensate for electron beam deflection variations.
When a user tunes the breathing compensation circuit so that dark images are as wide as bright images, he looks at one side of the screen and makes sure that dark images start at the same position as bright images. This tuning is enough as breathing effects are symmetrical. Indeed, a same electron beam current is deviated as much to the left as to the right.
Nevertheless, the applicant noticed that, in many CRTs, when tuning the breathing compensation circuit to adjust the left side of the image, another deflection effect appears when switching from a dark image to a bright image or conversely. When switching from a dark image to a bright image, while the position of the left side of the image no longer changes, the right side of the image shifts to the left. Conversely, the right side of the image shifts to the right when switching from a bright to a dark image.
Consequently, a purpose of this invention is to provide an electron tube display control circuit that overcomes the drawback described above.